This invention relates to electronic devices, and more particularly to a packaged power semiconductor device with a metal back plane that is electrically isolated from the terminals of the device.
Most power semiconductor devices like silicon-controlled rectifiers (xe2x80x9cSCRsxe2x80x9d), power transistors, insulated-gate bipolar transistors (xe2x80x9cIGBTsxe2x80x9d), metal-oxide-semiconductor field-effect transistors (xe2x80x9cMOSFETsxe2x80x9d), power rectifiers, power regulators, or combinations thereof, are assembled in packages that are not electrically isolated. That is, a metal tab, which typically forms the backside of the packaged device, is electrically coupled, e.g. soldered, to the semiconductor die within the packaged device. This places the backside of the package at the same electrical potential as the semiconductor die.
Power semiconductor devices are designed to operate at relatively high voltages, typically 30-1,000 V, or higher, compared to other electronic semiconductor devices, such as logic or memory devices. In a conventional packaged power semiconductor device, the backside of the package may be subject to these voltages under normal operation or if a device failure occurs. Additionally, the power semiconductor device may be exposed to voltages outside of the intended range during operation, which may electrically couple to the backside of the package.
The high voltages present at the backside of conventional packaged power semiconductor devices may damage other circuit components or may present a safety hazard to personnel operating equipment built with these devices. Voltages as low as 40 Volts can be a hazard to operators working with or on such equipment. Insulating pads or washers are typically used to electrically isolate the backside of the power semiconductor device from the rest of the circuit. In a typical application, the power semiconductor is mounted on a heatsink that is part of an electrical chassis at ground potential.
Heatsinking is important for power semiconductor devices because of the power dissipated by some devices during operation, and also because of the environment that the device may have to operate in. Power semiconductor devices are often used in applications that may get relatively hot, .such as in an engine compartment or in a factory. Thus, it is especially important to minimize the thermal resistance between the active device, which may be generating several Watts or even several kilowatts of power, and the environment, which may be an elevated temperature.
FIG. 1A is a simplified exploded view of a packaged power semiconductor device 10, an insulating pad 12, a heatsink 14, and a screw 16. The screw is used to attach the semiconductor device and the insulating pad to the heatsink. The semiconductor die (not shown) is attached to a metal tab 21, leads 18 are electrically coupled to the terminal of the die, and the assembly is then encapsulated with encapsulating material 20. The encapsulating material is typically epoxy, plastic, rubber, silicone, or similar materials and is molded, cast, or otherwise formed over the die and related structures.
The heatsink 14 is usually metal, and the insulating pad 12 is typically made of an insulting material such as silicone rubber, mica, or ceramic, and may be in the form of a washer or other shape, rather than a pad. It is desirable that the insulating pad 12 provide electronic isolation between the backside 22 of the power semiconductor device 10 and the heatsink 14 while also providing good thermal coupling to the heat sink.
FIG. 1B is a simplified view of the power semiconductor device.10 and the insulating pad 12 mounted to the heatsink 14 with the screw 16. The screw can be made of an insulating material, such as nylon, or additional insulating washers and/or sleeves can be used to isolate the power semiconductor device from the heatsink. In some applications, it may be necessary to provide enhanced heatsinking. Soldering the power semiconductor device to the heatsink will provide superior thermal coupling. However, the heatsink would typically need to be isolated from the rest of the circuit or chassis, and may require a shield to prevent a technician from inadvertently touching an electrically xe2x80x9chotxe2x80x9d heatsink, which could be at a lethal voltage.
Adding electrical isolation between a power semiconductor device and the heatsink reduces thermal coupling to the heatsink and requires additional parts and assembly. Providing electrical isolation between a heatsink and the chassis similarly involves additional parts and assembly. The additional parts and assembly not only increases cost, but also provides an opportunity for human error, namely, omitting the isolation altogether. Additionally, some isolation pads are fragile, and may crack, tear or otherwise be damaged during assembly or subsequent use. Omission of, or damage to, the isolation pad may cause the isolation to fail and expose an operator to dangerous voltages.
Therefore, it is desirable to mount packaged power semiconductor devices to heatsinks or other circuit components without requiring additional parts or labor. It is further desirable that the packaged power semiconductor be mounted in a reliable and safe fashion, and that good thermal coupling between the packaged power semiconductor device and the heatsink be achieved. It is still further desirable that any such packaged power semiconductor device be adaptable to be retrofitted into existing applications.
The invention provides a packaged power semiconductor device having electrical isolation between the potential of a die inside the package and a metal back side of the package. A power semiconductor die is mounted on a direct-bonded copper (xe2x80x9cDBCxe2x80x9d) substrate. The die may be mounted using solder or other die-attach means. A lead of the package is also soldered to the DBC substrate. In some embodiments, all leads are soldered to connecting pads on the DBC substrate.